1. Field of the Invention
The present invention relates to a semiconductor laser element, and more particularly, to a semiconductor laser element of a window structure for enhanced light output power.
2. Related Background Art
Recently, semiconductor laser elements using InGaAlP compound semiconductors for wavelengths from 600 nm to 700 nm have advanced to practical use in DVD (digital versatile disk), and have been enhanced in output power to be available for use as write lasers for recordable or rewritable DVD-RAM and others. When such lasers are enhanced in output power, the issue of catastrophic optical damage (COD) arises. Therefore, many lasers of this type have window regions in their laser beam emission edges. The structure having such window regions is called a window structure.
FIG. 5 is a cross-sectional view of a conventional semiconductor laser element having a window structure. This laser element includes an n-type clad layer 202 of In0.5(Ga0.3Al0.7)0.5P, guide layer 203 of In0.5(Ga0.4Al0.6)0.5P, active layer 204 having a multiple quantum well (MQW) structure made of In0.5Ga0.5P/In0.5(Ga0.5Al0.5)0.5P, guide layer 205 of In0.5(Ga0.4Al0.6)0.5P, p-type clad layer 206 of In0.5(Ga0.3Al0.7)0.5P, and p-type contact layer 207 of GaAs that are sequentially formed on an n-type GaAs substrate 201. With the supply of a current injected from a p-side electrode 291 and an n-side electrode 292, the active layer 204 emits light from its emission region 232 toward the front edge 210 on the left and the rear edge 220 on the right when viewed on FIG. 5. Near the front edge 210 and the rear edge 220, a front window region 212 and a rear window region 222 shown by hatching are formed by diffusion of zinc from above in the figure. In these window regions 212 and 222, the well layers and the barrier layers of the active layer 204 are alloyed. The front edge 210 is coated by a low-reflectance film 211, and the rear edge 220 by a high-reflectance film 221. Cavity length of the high-power semiconductor laser element of FIG. 5 is 700 to 900 μm, which is slightly longer for a larger gain, and length of each window regions 212 and 222 in the horizontal direction of the figure is around 30 to 40 μm near the active layer 204. The well layers and the barrier layers of the active layer 204 are about 5 nm thick, respectively, and the number of well layers is about three.
In the element of FIG. 5, when Al is introduced into the In0.5Ga0.5P well layers in the window regions 212, 222 from the In0.5(Ga0.5Al0.5)0.5P barrier layers, bandgap of the well layers in the window regions 212, 222 is widened. As the bandgap is widened, the bandgap wavelength is shortened. Semiconductors, in general, do not absorb light of wavelengths longer than the bandgap wavelength. Therefore, the well layers of the active layer 204 in the window regions 212, 222, widened in bandgap and shortened in bandgap wavelength, are less likely to absorb light from the well layers of the active layer 204 in the emission region. As a result, optical damage is less likely to occur.
Heretofore, it has been believed that maximizing the bandgap of the window regions 212, 222 to the extent not adversely affecting the operation would be advantageous for better laser properties under the belief that the wider the band gap of the window regions 212, 222, the laser should be enhanced in reliability because of less light absorption and less optical damage in the well layers of the active layer 204 in the window regions 212, 222. Simultaneously, it has been believed that the laser should be enhanced in light output power.